This invention relates to apparatus for providing a rasterscan display representative of change of a first variable as a function of a second variable.
An oscilloscope is conventionally used to display a waveform representative of change of a first variable as a function of a second variable. It will be convenient in the following description to assume from time to time that the first variable is the magnitude of a measured quantity and that the second variable is time, so that the waveform represents magnitude of the measured quantity as a function of time, but it will be understood that the second variable need not be time or even related to time.
In a conventional analog oscilloscope, an electron beam repetitively sweeps horizontally across a cathode ray tube (CRT) screen creating a trace of glowing phosphor on the screen. When the magnitude of an input signal controls the vertical position of the beam during its horizontal sweep across the screen, the trace represents the behavior of the input signal as a function of time. A trace produced by a single beam sweep eventually fades out, but when the input signal is periodic and the beam sweeps are periodically initiated at similar points during successive cycles of the input signal, the beam traces out the same path during each sweep and a stable waveform is continually displayed.
The behavior of the input signal might not be precisely the same during each sweep cycle. When the screen phosphors are persistent enough to glow substantially longer than a single sweep cycle, the waveform display represents a time-weighted record of traces produced by several previous sweeps of the beam and conveys information regarding the behavior of the input signal during more than just the most recent sweep cycle.
In an analog oscilloscope, the time taken to complete a horizontal sweep is constant, and therefore the intensity of the trace depends on the length of the trace. The length of the trace depends on the frequency of the input signal, and therefore the intensity of the trace provides valuable information concerning the frequency of the input signal.
An operator might adjust the beam intensity so that a screen phosphor particle becomes brighter each time the beam strikes it, and then becomes dimmer until it is struck again. This persistence mode of operation helps reduce effects of transient noise on the waveform display inasmuch as vertical excursions of the beam due to transient noise produce only dim traces that quickly fade away, whereas the display of the underlying stable waveform remains bright.
In a vector digital oscilloscope the electron beam traces out vectors having their end points at discrete screen locations organized as an array of horizontal rows and vertical columns. In the event that the oscilloscope is used to display variation of signal magnitude as a function of time, each column represents a different sampling time interval and each row represents a different signal magnitude. Typically, an input signal is sampled and digitized and a succession of pairs of digital words is generated, one word of each pair representing the magnitude of the input signal and the other word representing sample time following a trigger event. The magnitude value of each pair is written into an acquisition memory at a location that depends on the associated time value, to form a waveform record. When a single acquisition is complete the contents of the acquisition memory can be used to create a stable display on the CRT screen.
A known rasterscan digital oscilloscope has a display memory in which the number of addressable memory locations is equal to the number of displayable pixels of the display screen. The address of each memory location has two components, one depending on the magnitude of a sample and the other on the time at which the sample was taken. The two components of the memory address correspond respectively to the X and Y components of the pixel address on the CRT screen. A display is formed on the CRT screen by scanning all the pixels in accordance with a raster pattern and illuminating the pixels selectively, depending on the contents of the corresponding memory locations. If each memory location is capable of storing a single bit of data, the beam is turned on if the value of the bit is logical 1 and is held off if the value is logical 0.
If each memory location is able to store more than a single bit of data, each pixel can be illuminated with multiple gray scale levels. For example, each memory location might be able to store four bits, representing off and 15 gray scale levels. In use of an oscilloscope having such a memory, the content of a memory location is read from the memory when a sample pair having the same combination of magnitude and time components is received, and the value stored in the memory is progressively increased to a maximum of decimal 15. Moreover, from time to time the content of each memory location is read and is progressively decreased to a minimum of 0. In this manner, it is possible to increase the information content of the display and emulate the persistence feature of an analog oscilloscope.
U.S. Pat. No. 4,223,353 (Keller) discloses a digital rasterscan display device in which the decay can be controlled to be a function of time only or a function of both time and rate of accumulation of data. If the accumulation rate is low, the operator may change over to the time-based decay.
In the case of a single-valued signal, i.e. a signal of which the magnitude component is the same on each acquisition for a given time component, the information content in a display of the signal waveform is built up very quickly and it takes only a few acquisitions for the pixels that are illuminated to reach full intensity. On the other hand, other signals, such as a TV line signal, may have different magnitudes at a given time in successive acquisitions and therefore it can take several acquisitions to build up information content. It is clear therefore that a decay rate based on acquisition rate or time only will not provide a display with optimum information content.